Atmosphere 3,150 Gt CO
Net atmospheric growth 17 Gt CO
Energy 13 Gt CO
Buildings 3 Gt CO
Industry 7 Gt CO
Transport 7 Gt CO
Agriculture 4 Gt CO
Land use change 5 Gt CO
Land sink 12 Gt CO
Ocean sink 9 Gt CO
4
1
2
3
10
5
Vegetation 2,000 Gt CO
8
9
Biosphere 15,000 Gt CO
Soil 5,500 Gt CO
Hydrosphere 148,000 Gt CO
Fossil fuels Reserves 3,100 Gt CO Resources 45,000 Gt CO
7
6
Seawater 139,000 Gt CO
Lithosphere 285,000,000 Gt CO
1 Chemicals from CO 2 Fuels from CO Pathway
Sediment 6,400 Gt CO
Net ows
3 Products from microalgae 4 Concrete building materials 5 CO –EOR 6 Bioenergy with carbon capture and storage
Closed pathway
7 Enhanced weathering 8 Forestry techniques 9 Soil carbon sequestration techniques 10 Biochar
Cycling pathway
Open pathway
Figure 3 Stocks and net flow of CO 2 including potential utilisation and removal pathways Source: Adlen and Hepburn, 2019
that permanently remove atmospheric CO 2 or permanently displace CO 2 emissions. In mineral carbonation, CO 2 reacts with a metal oxide such as magnesium or calcium to form carbonates (Metz, et al ., 2005) (Li, et al. , 2013). The process encompasses a series of reactions that can take place in a single- or a multi-step process (three main reactions), also known as direct and indirect carbonation, respectively (Metz, et al ., 2005). In a single-step process, the extraction of the metal from the mineral matrix and the carbonate precipitation occur simultaneously in the same reactor. Direct carbonation takes place under high pressure conditions in either dry or aqueous media (Huijgen & Comans, 2007). As an example, the overall carbonation reaction is illustrated below:
motivation, together with some degree of climate mitigation potential. Adlen and Hepburn estimated that at the top end over 10 GtCO 2 a year or a quarter of global emissions (40 GtCO 2 ) could be utilised for less than $100 per ton. Most of that utilisation is associated with medium- or long-term storage in open and closed pathways. However, there are several problems and challenges to overcome before such large utilisation could be achieved. Because of these issues, whether the implementation of a CO 2 utilisation technology will be climate beneficial will depend on a host of factors. The most important of these are: (a) Energy source: CO 2 utilisation technologies can be energy intensive. This energy needs to be renewable: either directly from the sun or via renewable technologies. (b) Scale: in order to make an appreciable difference to the global flows of CO 2 , pathways need to have the potential to scale quickly and (c) Permanence: the most impactful technologies will be the ones
Mg 3 Si2O 5 (OH) 4 + 3CO 2 3MgCO 3 +2SiO 2 + 2H 2 O
The carbonation reaction is an exothermic reaction releasing enough heat to make the whole
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